1. Field of the Invention
This invention relates to an all-optical modulator for modulating the output power of a high-power optical pump source. More particularly, the invention relates to an all-fiber modulator wherein the output power of a CW fiber laser is modulated by a low-power laser, such as a diode laser. The modulator comprises two optical couplers which are preferably wavelength-selective and interconnected by a nonlinear single mode optical fiber with a high Raman gain. The likelihood of optical damage in the modulator is greatly reduced due to its all-optical design.
2. Description of Related Art
High power laser sources, in particular solid state lasers such as diode-pumped Nd:YAG and fiber lasers doped with Yb, are known in the art and are increasingly used for materials processing and marking as well as for printing and graphic arts applications where rapid modulation of the high-power optical output is essential. The highest attainable modulation frequency in such lasers through external modulation of the diode pump source is determined by the fluorescence lifetime of the excited state which is in the range of microseconds to milliseconds, making modulation at frequencies above 1 MHz increasingly difficult and inefficient. Consequently, most solid state and fiber lasers emit a continuous (CW) optical output which is subsequently modulated externally by electro-optic or acousto-optic modulators known in the art. This approach reduces the overall efficiency of the system, since the portion of the output energy which is not transmitted by the modulator to the target, is deflected to a "beam stop" and absorbed in the form of heat.
Moreover, electro-optic or acousto-optic modulators require expensive drive electronics and careful alignment with the optical beam. In addition, the optical characteristics of these modulators are temperature-dependent due to the temperature dependence of the physical parameters of the crystals, such as their linear dimensions, refractive index, sound velocity, etc.
With the foregoing in mind, it is therefore an object of the invention to provide an all-optical modulator for high-power laser sources, in particular for fiber lasers, which does not require complicated beam alignment and which is stable with changes of the environmental parameters. It is a further object of the invention to provide a modulator of this type which has a low insertion loss and which can be modulated at frequencies above 1 MHz. The modulator should preferably also have a high extinction ratio and a high damage threshold.